Telephone trunking system.



No. 818,527. PATENTED APR. 24, 1906.

W. W. DEAN.

TELEPHONE TRUNKING SYSTEM.

' APPLICATION FILED JULY 14, 1902.

' 11g at i W I W IIIHIWIIIPH Q i -2- 3; avg H R fig W 2 UNITE STATESPATENT l FFIQEO WILLIAM WV. DEAN, OF CHICAGO, ILLINOIS, ASSIGNOR TOKELLOGG SWVITCHBOARD AND SUPPLY COMPANY, OF CHICAGO, ILLINOIS, A

CORPORATION OF ILLINOIS.

TELEPHONE TRUNKING SYSTEM- Specification of Letters Patent.

Patented April 24;, 1906.

Application filed July 14, 1902. Serial No. 115,406.

To (/LZZ/ whom it may concern:

Be it known that I, WILLIAM W. DEAN, a citizen of the United States,residing at Chicago, in the county of Cook, State of Illinois, haveinvented new and useful Improvements in Telephone Trunking Systems, ofwhich the following is a specification.

In an application filed by me June 28, 1902, Serial No. 113,581, I haveshown, described, and claimed a trunking-circuit and apparatus forconnecting together subscribers lines of the well-known Dunbar ortwo-wire type.

The present invention relates to the same general subject-matter as thatof the abovementioned application, and while many of its features may beemployed in other relations it is intended particularly for theinterconnection of subscribers lines of the abovementioned type.

In the present invention one object is to provide a trunk-relay which isactuated when the connection is established at the outgoing end of thetrunk in a manner to prevent the operation of the disconnect-signal atthe incoming end and to operate the guard-signal at said end in case thecalled-subscribers line has not been connected with by the incoming endof the trunk and to still maintain the armature of said relay inactuated condition during conversation, although its winding is deprivedof operating-current.

A further object is to provide, in connection witha trunking system ofthe kind described, a polarized trunk-relay and means whereby when aconnection is established with the trunk-circuit at the outgoing end thetrunkrelay armature is actuated in one direction and when the connectionis severed at said end the trunk-relay is actuated in the reversedirection. Further objects will be apparent from the description andclaims.

My invention is illustrated in the accompanying drawing, in which thesame reference characters designate like parts throughout, and in whichthe figure is a diagram showing two subscribers lines leading to twodifferent central oflices and the trunking connection between saidoflices to enable the subscribers to be connected together forconversation.-

In the figure, L designates one of the number of subscribers linesterminating at the central office C. This line extends in two limbs 2and 3 from the subscribers station A to the said central ofiice C, whereit is fitted with suitable answering and multiple jacks orconnection-terminals J J At the subscribers station the usual telephoneinstruments are provided and consist of a transmitter 4, a receiver 5, aringer or call-bell 6, and the condenser 7. The switch-hook 8 is adaptedin its normal position and when the receiver is placed thereon to openthe circuit through the transmitter 4 and receiver 5 and to close itthrough the call-bell 6 and condenser 7. A retardation-coil 9 isconnected at the substation when the hook is raised in parallel with thereceiver 5 and condenser 7 to provide a path transparent to steadycurrents, but opaque to voice-currents. The line conductors 2 and 3include, respectively, the armatures or springs 10 and 11 of the cut offrelay R, which is permanently legged to ground from the sleeve-contacts3' y' of the jacks J J The armature or spring 10 is normally connectedwith ground through the conductor 12, while the opposite armature 11 isnormally connected with conductor 13, containing the line-relay R forthe line, said conductor leading to the live or ungrounded pole of thecentral battery B, the opposite pole of which is grounded. Theline-relay R controls, through its armature, the circuit of theline-signal S. The forward contacts 14 and 15 of the cut-off relay R areadapted to connect the jacks and the switchboardsection of the line withthe external line-circuit when the said cut-off relay is energized.This, it is to be understood, is a typical twowire line-circuit and isthe particular kind with which I prefer to use my trunk-circuits,although I do not wish to so limit the use of the invention in allrespects.

At the central oflice C the usual operators cord-circuit is provided,said cord-circuit having an answering-plug P and a calling-plug P eachbeing provided with a tip-contact p and a sleeve-contact p adapted toregister with like contact-surfaces j and j in the jacks of the linewhen the plug is inserted therein. The tip-contacts of the plugs areconnected together by means of the tip-strands t and t of thecord-circuit and the interposed condenser c, while the sleeve-contactsare likewise connected by the sleeve-strands s and s and the interposedcondenser A conductor 16 extends from the grounded pole of the battery Bto the tip-strand t and contains the winding of the tip-supervisoryrelay r associated with the answering-plug P, while the live pole of thebattery is connected, by means of conductor 17, with the answeringsleeve-strand s and includes the winding of the sleeve supervisory relayr These relays r and r serve to control the circuit of thesupervisorylamp S associated with the answer ing-plu P, the former relayserving to normally cTose said circuit of the lamp, while the latterrelay normally opens the same. The battery B is associated with theopposite or calling end of the cord-circuit and is connected byconductors 18 and 19 with the tip and sleeve strands t and 8respectively, the former conductor including the winding of thesupervisory relay 1, while the latter conductor includes the similarrelay r These relays serve in a manner similar to the relays r and r tocontrol the circuit of thesupervisory lamp S associated with thecallingplug P The relays r and r are conveniently referred to as thetip-relays and relays r and r as the sleeve-relays, since they areconnected with and operated by current fiow ing over the correspondingstrands of the cord-circuit. The tip-strand t of the calling and testingplug P is normally open through the extra spring or movable contact 20of the sleeve-relay r but is adapted to be closed by the said spring andits forward contact 21 when the relay is energized. The back contact 22of this extra spring 20 is connected, through the medium of conductor23, with one side of the high-resistance and high-impedance relay 1known as the test-relay, the other side of which is grounded. Thearmature of this relay is likewise grounded, and its forward contact isconnected with the primary winding of the operators inductioncoil 25 bymeans of conductor 24. The other end of the said primary winding isconnected, through the medium of conductor 26, to the live pole of thebattery B ,-a retardation-coil 27 being included in this circuit. Theoperators transmitter 28 is joined on one side to conductor 26,intermediate her primary coil and the coil 27, and on the other sidethrough conductor 29 to ground. A condenser c is included in theoperators local circuit to facilitate talking and is connected as shown.The secondary winding of her induction-coil 25 is adapted to beincluded, together with her receiver 30 and a condenser'c", in a bridgebetween the strands of the cord circuit by means of any suitable key andindicated diagrammatically by k 7c. Branch connections 31 and 32 leadfrom opposite sides of the receiver and secondary to an order-key kconnected with an order circuit or wire 33, leading to the ofiice C Agrounded alternatingcurrent ringing-generator g is adapted to beconnected through a resistance-lamp with the tip-strand t of thecord-circuit by means of the tip-ringing key-spring le the sleevecontact 16 of said key'being connected with battery-lead 26 through wire26 and resistance 26 A trunk-circuit is shown extending bei tween thecentral oflices C and O said trunkcircuit being fitted at the outgoingend with multiple jacks, such as J 3 and J with a relay 34 legged toground from the sleeve side of the circuit. The battery B which may beeither a separate battery, as shown, or the same battery as B or B isconnected, through a resistance and the normal contacts of said relay 34to'the tip side of the trunk. At the opposite end of the trunk apolarized trunkrelay 7 of high resistance is connected between thewindings w and w of the repeating-coil W, which are connected with theop posite trunk-limbs 35 and 36. Battery B is so connected as tomaintain the armature of the polarized relay 9 in the position shown andconnected to ground. The opposite windings w and w of the repeating-coilW are connected together with an interposed condenser between thetip-strand t and the sleeve-strand s of the trunkcord, which strandsterminate in the corresponding contacts p and p of the trunk-plug Prelay 1" is legged to ground from the tip-strand t of the trunk-circuitby conductor 36 and controls, through its forward contacts, a shortcircuit composed of conductors 38 and 39 of the high-resistancepolarized trunk-relay W. A sleeve-relay 1" is connected on one side withthe live pole of the battery B by means of conductors 40 and 41 and onthe other side with sleeve-strand 8 This incoming end of thetrunk-circuit is provided with a testing apparatus similar to thatdescribed for the The tip- 9 cord-circuit at the central oflice C. Thefor ward portion of the tip-strand t is connected with spring 42 of thesleeve-relay 1*, while the other portion is connected with the forwardcontact of said spring. The back contact of spring 42 is joined byconductor 43 with the common high-resistance and impedance test-relay rthe opposite terminal of which is grounded. The armature of thistest-relay serves when actuated to ground one side of the primarywinding of the operators induction-coil 44, which winding is connectedon the other side with a conductor 45, containing the resistance andimpedance coil 46 and joined to conductor 47, leading directly to thelive pole of the battery B The operators transmitter 48 is in aconductor 49, leading from conductor to ground. A condenser c isconnected between conductor 49 andthe side of the primary that isadapted to be grounded. The operators receiver 50, together with thesecondary of her 33, leading to the central oflice C. The forwardcontact of spring 51 of sleeve-relay 1". is connected by conductor 52with the conductor 40, leading from the live pole of generator B". Thespring 5 1, which is normally grounded, is connected by conductor 53,leading to point 54, and thence by conductor through the ringing-lamp Sto the normal contact of the grounded armature of lockingrelay 5. Thelocking-relay r is connected in a conductor 56, extending from point 54to the forward contact of grounded spring 57 of tip-relay W. The forwardcontact of the locking-relay armature is joined by conductor 58 withconductor 56 at point 59 The guard and disconnecting-lamp S areCOIll'lGGl'r ed between the point 54 and the armature of polarizedtrunk-relay 1, the lower and normal contact of which is grounded, asshown, and the upper and normally open contact of which is connected, bymeans of conductor 40, to the live pole of the battery B This incomingend of the trunk is equipped for selectively ringing subscribers upon apartyline. It comprises the ringing-key contacts k k and 7C5 7?, placedin the strands of the trunk-cord and adapted When actuated to suitablyconnect the alternating-current generator g with the plug P. Thisringing-generator comprises the armature 60, the grounded brush 61, thebrush 62, from which the alternating current is directed through resistance-lamp 63 to tip-key spring is when depressed, and brush 64 fromthe split commutator-ring, which is adapted to deliver a current ofnegative pulsations only through lamp 65 to the sleeve-key k when thekey is operated. The sleeve-contact of key k when operated connects thesleeve of the plug to battery-lead 47 through resistancecoil 66 tomaintain the cut-off relay of the line operated during ringing. Thesubscribers line shown in connection with this oflice is of theparty-line type and is connected to subscribers D and D The apparatus atthese stations is similar to that at the sta tion A and are likewiseindicated, except that the bell 6 at station D is connected between thesleeve-conductor 3 and ground, while at station D the bell is connectedbetween the tip-conductor 2 and ground. The same type of normallydisconnected acks J J line-relay R cut-off relay R, and the line-signalS are employed and are designated by the same reference characters. Thebattery B furnishes current for the operation of these various relaysand signals, as well as for talking.

J J and J J are respectively busyback and dont-answer jacks associatedwith the incoming trunks. These circuits comprise an induction-coil 67,having its primary in circuit with a rapidly-rotating circuit-breaker 68and the battery B this battery being preferably the same battery as Bthough shown separately for convenience.

The secondary of the induction-coil 67 is con nected in circuit with therapid circuit-interrupter 69 and the more slowly operating interrupter70. From interrupter 69 a conductor 71 leads to the tip-contacts ofbusy-back jacks J J 5 through a fifty-ohm resistance-coil 72, whileinterrupter connects by con.- ductor 73 through resistance-coil 74 tothe tips of the dont-answer j acks. The sleeves of both sets ofjacks areunited by conductor 75 with the grounded pole of the battery. Thecircuit-breaker 68 causes induced currents in the secondary circuitswhich are interrupted at short intervals by breaker 69 and at longerintervals by breaker 70 to cause distinctive tones at the acks J 5 and Jwhereby when a trunk-plug is inserted in one or the other jack acorresponding signal is transmitted to the waiting subscriber.

In tracing the circuits hereinafter the plan for pointing out the pathfor current from the live or ungrounded pole of the battery to theground only will be followed for convenience, it being understood thatthe returncurrent will seek the grounded pole of the battery and thatthe circuit will therefore be complete.

The operation is as follows: The A subscriber desiring a connection witha subscriber located at another exchange removes his receiver from thehook, and thereby closes a path for current between the limbs 2 and 3 ofhis line through the transmitter 4 and retardationcoil 9, the condenser7 and receiver 5 being connected in parallel with the retardation-coil.The closing of this circuit permits current to flow from the battery Bthrough conductor 13, line-relay R spring 11 of cut-off relay R, limb 3of the telephone line, through retardation-coil 9 and transmitter 4 atthe substation, and thence over limb 2 back to the central oflice andspring 10 of cut-off relay R, through conductor 12 to ground. Theline-relay R is thus operated and closes the circuit of the signal-lampS to ground from the conductor 13. The oper ator upon seeing theline-signal exposed i11- serts the answering-plug P into theanswering-jack J of the subscribers line and connects her telephone 30with the cordcircuit by depressing the listening-key 7c 76 to receivethe order from the subscriber. The insertion of the plug P closes acircuit through the cut-off relay R from the live pole of the battery Bthrough conductor 17, sleeve supervisory relay r sleeve-strand s,sleeve-contacts p and of the plug and jack, thence'through the windingof the cut-off relay R to ground. The operation of this re laydisconnects the armatures 10 and 11 from the ground-wire 12 and from thebatterylead 13 and connects them through the forward contacts 14 and 15of said armatures to the normally disconnected jacks J J The operationof the supervisory relay r over the '13 furnishes current over themetallic telephone-line for the operation of the supervisory relays andfor conversational purposes. The operators transmitter 28 is receivingcurrent at this time from the battery B over the conductor 26, throughretardation- .coil 27, thence through the transmitter 28 and overconductor 29 to ground. When sound-waves strike the transmitter 28, thecurrent flowing therethrough is varied, which causes a variation of thecharge in the con denser c and a corresponding surge of current throughthe primary of her inductioncoil 25. These induce voice-currents in thesecondary, which are transmitted to the line, and the operator istherefore able to converse with the calling subscriber. Upon learningthat a subscriber in the exchange C is desired, the A operator depressesthe orderkey k to connect her telephone with the order wire or circuit33 and repeats the number of the wanted subscriber to the B operator,whose head telephone 50 is permanently connected with the saidorderwire. The latter operator designates to the A operator the trunk tobe used for the connection and proceeds to test the condition of thewanted line with the tip of the plug of the incoming trunk named. If theline is idle, no click will be received, but if busy the sleevecontactsof the jacks of the line will be connected with the live pole of abattery through the sleeve contact and strand of the inserted plug. Acomplete circuit will therefore be established from the sleeve-contactof the tested jack, which it is assumed is connected with the live poleof the battery, through. the tip of the plug P tip-strand 25 spring 42,and back contact of sleeve-relay r to conductor 43, through the commonhigh-resistance and impedance test-relay or responsive device 1" toground. This testrelay will therefore operate and ground, through itsarmature and front contact, one side of the primary of the operatorsinduction-coil 44, the other side of which is connected by conductor 45through resistance 46 and conductor 47 to the live pole of the battery BThe completion of this circuit will permit a flow of currenttherethrough which will induce a current in the secondary of the saidinduction-coil and produce a click in the receiver 50 of the operatorsset.

The operators receiver will respond to this click at all times, eventhough the order-circuit should be open at the A exchange, since theelectrostatic capacity of her circuits and that of the order-circuit isample for the purpose of producing the click.

In the threewire systems, where the testcircuits are insulated from thesubscribers line, there is no danger of varying the current flowing inthe line to the annoyance of the subscriber during testing; but in thetwowire systems, where the testing contacts are connected directly withthe talkingcircuit, some means is necessary, or at least desirable, toprevent noticeably varying the current upon lines connected forconversation when testing their condition at other sections of theswitchboard. This is accomplished in the circuit described by means ofthe test-relay r which is of such high resistance as to vary the currentupon the tested line but slightly and which is also of high impedance,so that its current will build up slowly, to thereby prevent a suddenvariation of current on the line. The subscriber or subscribers whoselines are tested are therefore undisturbed by a sudden or largevariation of current through their instruments. It is apparent that anyother suitable test-responsive device or indicator than the relay r canbe employed and that the operators telephone can be used in place of thesame or in the circuit with a retardation-coil; but the arrangementdescribed is preferred. This relay is preferably common to all theincoming trunks at one operators position, as indi cated by thebranching lines at 43. Upon finding the line idle the plug P is insertedin one of the multiple jacks of the wanted subscribers line.

The A operator having received the number of the trunk to be usedinserts the callingplug 1? of her cord-circuit in the jack J of thetrunk at her section. The insertion of the plug P closes the circuitfrom the live pole of the battery B through the sleevestrand and throughthe relay 34 to ground, thus. operating the same and cutting off thebattery B from the tip side of the trunk-circuit and at the same timepermitting the negative current to flow over the sleeve side of thetrunk and through the trunlerelay r in the usual way. The trunk-relay istherefore reversed; but on account of its high resistance the tipsupervisory in the A-cord circuit is not at this time operated, forwhich reason the supervisory signal S is operated. The insertion of theincoming trunk-plug into the jack of the wanted line closes a circuitfrom the live pole of the battery B over conductors 40 and 41, throughsleeve-relay r, and thence over the sleeve-strand s of the trunkcord,and thence through the sleeve-contacts of the plug and jack to ground,operating the cut-off relay. The sleeve-relay is also operated anddisconnects the test-relay r and completes the tip-strand t of thetrunk-cord. This relay also completes the circuit of the ringing-lamp Sfrom the battery B over conductor 40, conductor 52, forward contact andspring 51 of sleeve-relay, conductor 53, to point 54, and thence throughthe ringinglamp S over conductor 55, to the back contact and armature oflocking-relay 1", to ground. This lamp is therefore lighted andindicates to the operator that the called subscriber has not removed histelephone from its hook. The operation of the trunk-relay r at this timeopens, through its armature and back contact, the circuit of the guardand disconnecting-lamp S to thereby prevent its operation. Upon theinsertion of the trunk-plug P the B operator rings the wantedsubscriber. If the subscriber D is the one desired, the ringing-key k kis depressed, which throws ringing-current upon the sleeve-strandconnected with the plug P and operates the subscribers bell over thefollowing path: from ground at the generator g brush 61, armature of thegenerator, brush 64, through resistance-lamp 65, key k thence over thesleeve-strand of the cord to the plug P over the sleeve-limb 3 of thetelephone line to the station D, and thence through condenser 7 and bell6 to ground, the receiver 5 being at this time upon the hook 8, andtherefore completing the circuit between the condenser 7 and bell 6.

It will be noticed that just previous to the depression of theringing-key k the cut-off relay R was receiving a negative currentthrough its coils from the battery B At the instant the ringing-key k isdepressed this path of current from the battery 13 is broken; but a newpath is immediately established through the relay R from the negativesource of pulsating ringing-current. The relay R is so constructed as tobe maintained in continuous operation when this negative pulsatingcurrent is passing through its windings, and as pulsations are used ofthe same direction as the current furnished by the battery B the relay Ris prevented from even momentarily releasing its armatures at the momentthe ringing-key was depressed.

The bell 6 at the subscribers station may be of the ordinary polarizedtype, as the presence of the condenser 7, which is connected directly inthe ringing-circuit, permits the operation of such a bell from thepulsating source of ringing current. The tip-spring of ringing-key isgrounded at this time to provide a path to ground for the return-currentif the subscriber should respond during ringing. Should the subscriber Dbe the one desired, the ringing-key k 76* is depressed, which serves toconnect the generator 9 with the tipstrand of the cord-circuit tooperate the bell at the station D The ringing-circuit may be traced asfollows: from ground at the generator 9 brush 6], armature-winding 60 ofthe generator, brush 62, through resistancelamp 63, spring k in thetip-strand of the cord-circuit, thence over the tip-strand and tipconductor 2 of the line to the station D and thence through condenser 7and normally closed contacts of the hook 8, and through hell 6 toground. The bell is thus operated. To maintain the cut-off relay Roperated during ringing, the inner contact of the sleevespring k isconnected through a resistance-coil 66 with the battery-lead 47,extending to the live pole of the batteryB This prevents thedenergization of cut-off relay R while ringing.

It is apparent that the arrangement of the ringing-keys is such thateither key may be used to call a subscriber at whose station theapparatus is connected in the manner shown at A, which shows thearrangement which I preferably use where but one subscriber is placedupon the line. In this case the ringing-circuit would be complete overthe subscribers metallic circuit, the current returning to thegroundedside of the ringinggenerator by means of the grounded key 16 orbattery-contact of the ringing-key 16 The subscriber having been calledresponds by taking up his receiver, and thereby closes a path forcurrent between the limbs of his line from the main battery B overconductors 40 and 41, sleeve-strand s of the incoming end of thetrunk-line 3 of the telephoneline, thence through retardation-coil 9 andtransmitter 4 at the substation, thence over limb 2 of thetelephone-line back to the central ofiice and over tip-strand t of thein coming end of the trunk including spring 42 and its forward contactof sleeve-relay 7'", through conductor 36 and tip-relay r to ground.This relay responds to the current now flowing through it and closesthrough its contact 37 the short circuit of the highresistance polarizedrelay 1, consisting of conductors 38 and 39. This relay being polarized,however, the armature thereof does not return to normal position, butremains in po sition with its upper end against the back contact. Theresistance of the trunk-relay r being now removed, the tip-relay r ofthe A cord receives sufficient current from the battery B to operate thesame, and thereby open the circuit of the supervisory signal S thusindicating to the operator that the wanted subscriber has responded tothe call and that the parties are in condition to converse. The closingof spring 57 of tip-relay 1" upon its forward contact, however,completes the circuit of the locking-relay 1", as follows: from the livepole of the battery B conductors 40 and 52, front contact and armature51 of sleeve-relay r conductor 53 to point 54, thence through thewinding of locking-relay r and over conductor 56 to the forward contactof spring 57 of tip-relay r and to ground. The locking-relay 1"therefore operates and opens through its armature and back contact thecircuit of the ringinglamp S which is extinguished and indicates to theoperator that the called subscriber has responded to his ring. Theclosing of the armature of the locking-relay 1" upon its forward contactcompletes a locking-circuit for said relay from the point 59, throughconductor 58, the forward contact, and. armature of the relay to ground.This circuit and relay are therefore locked and. the ringing-lamp Slocked out during the remainder of the connection and can only bereleased when the sleeve-relay r is deenergized, which occurs when theplug P is pulled out. Should the tip and sleeve contacts of the plug becrossed when the plug is inserted in a jack and the locking and otherrelays operated, they will be immediately released by the opening of thesleeve-strand in ringing and no confusion would result therefrom. Thesubscribers are now connected together for conversation and thesupervisory signals at both exchanges are extinguished. The battery B isfurnishing current over the answering end of the A-cord circuit to the Asubscribers line for talking purposes and for the operation of thesupervisory relays, the battery B is furnishing current over thetrunk-line for the operation of the supervisory relays associated withthe answering end of the A-cord circuit, and the battery B is sendingcurrent over the incoming end of the trunk and to the substation D tofurnish current for talking and for the operation of the several relays.The voice-currents are transmitted inductively from the subscribers lineL to the trunk-circuit through the condensers c and c in the A-cordcircuit and from the trunk-circuit to the incoming end thereof and thesubscribers line connected therewith through the medium of therepeatingcoil W. The condenser 0 between the windings w and w" of therepeating-coil at the incoming end of the trunk prevents the flow ofsteady current therethrough, but permits a free passage of thevoice-currents. The relay 34 by its retardation prevents leakage ofvoice-currents.

At the termination of the conversation the subscribers return theirreceivers to the hooks, with the result, in the case of subscriber A,that the tip supervisory relay connected with the A cord is denergizedand closes through its armature and back contact the circuit of thesupervisory signal S thus indicating that fact to the operator. When thesubscriber D han s up his telephone, current is cut off from themetallic line in the same way, and the tip-relay is thereforedeenergized andthe short circuit about the trunk-relay is opened.

The current in the trunk-circuit must now flow through thehigh-resistance trunk-relay, which permits the tip-relay r in the A cordto return to normal position, thus lighting the signal-lamp S The Aoperator observing that both subscribers have hung up their receiverstakes down the connection, thus restoring all parts at her ofiice tonormal position and permitting the battery B to put negative currentupon the tip side of the trunk, as at first described, to thereby causethe trunk-relay r to throw its armature into normal position. Ittherefore closes the circuit of the disconnecting-lamp S which circuitincludes conductors 40, 52, and 53, to the point 54, thence through thelimb and armature of the trunk-relay to ground. The lighting of thislamp indicates to the B operator that the A operator has disconnectedher cord-circuit from the trunk and that the plug P should be withdrawn.Upon withdrawing this plug the sleeve-relay r is denergized and thecircuit of the signal-lamp S is opened at the front contact of thearmature 51. The opening of this circuit also deprives the locking-relayof current, the armature of which returns to normal position. Thedisconnect ing-signal S serves also as a guard-signal in case the Aoperator inserts the calling-plug P of her cord-circuit into a wrongjack, for the reason that the trunkrelay armature will be thrown to itsabnormal position, thus closing a circuit from the said lamp, asfollows: from battery-lead 40 through the said armature and lamp andthence over conductor 53 to spring 51 and back contact of sleeve-relay rto ground. This will indicate to the B operator that a mistake has beenmade.

In case the wanted subscribers line is found busy or in case thesubscriber having been called does not respond the plug P is inserted inone of the jacks J or J G and the corresponding signal is transmitted tothe waiting subscriber. Upon inserting the plug P in one of the jacks Ja path for current is pro vided from the battery B through thesleevestrand of the trunk-cord, sleeve-contacts of the plug and jack,and .over conductor to ground. This will serve to operate thesleeverelay 1"- to complete the continuity of the tipstrand t of thetrunk-cord through its spring 42 and the front contact. Upon thecompletion of the strand at this point the tip-relay r is provided withcurrent from the live pole of the generator 13*, through the secondarywinding of the induction-coil 67, through circuit-breaker 69, and thenceover conductor 71 through resistance 72 and the ti'pcontacts of the jackand plug and over the tipstrand and through the relay r to ground. Thiscircuit is interrupted at the breaker 69, so that the relay r isalternately energized and deenergized to correspondingly make and breakthe short circuit of the trunk-relay 1, which is thereforesimultaneously removed from and inserted in the circuit of the outgoingend of the trunk. This causes the intermittent operation of thetip-relay r in the A-cord circuit to effect the flashing of thesupervisory signal S, whereby the A operator is notified that the calledsubscribers line is busy. The flashing of this light of coursecorresponds to the rapidity of the breaking of the circuit at breaker 67or 70, which indi cates to the operator whether the line is busy or doesnot answer. The subscriber is also notified by the tone caused in hisreceiver.

The 0rd er-circuit 33, as is indicated by the branching lines 33connects the B operator with several A operators either at the same ordifferent exchanges. The A operator is also provided with a plurality ofkeys k to connect herself with the different B operators. The busy-backor dont-answer apparatus is common to the various sections of theincoming trunlcboard, as is indicated by the branching lines.

Although I have described the method of making connections through themedium of the order-circuit, I do not wish to so confine the invention,for it is apparent that the A operator is able by means of her testingapparatus to test the condition of the several trunks before her untilan idle trunk is found. Upon so testing and finding an idle trunk shemay insert the calling-plug P of her cord-circuit, with the result thatthe trunk-relay 1 cperates and lights the lamp S which lamp has beenpreviously termed the guard-lamp, but in this method of operation itwould act as a calling-signal from the A operator to the B operator. TheB operator upon seeing the signal S would depress her listening-key 7c7r, which would connect her receiver in series with the repeating-coilwindings w and w", these receiver connections being indicated in dottedlines in Fig. 1. She is therefore able to communicate with the Aoperator. Upon learning the desired connection the B operator completesthe same, as before described. It will also be understood that had thesubscriber A called for a connection with another subscriber in the sameexchange the line of that subscriber would have been tested in theordinary manner by means of a cord-plug P and the test-relay r, and thesubscriber would have been called by the ringing-generator g.

The ground connections heretofore referred to, it will be understood,may and in practice usually are one and the same, or they may be thecommon office-return, although it is found by experience that it ispractically necessary to place a dead-ground upon one side of the commonbattery. It is also apparent that the other pole of the batteries may begrounded, so far as the operation of the system is concerned, the onlyvariation necessary from that shown in the drawings being to remove thepresent ground connections and replace them by other conductors andreplace the ground upon the opposite pole of the battery.

WVhile I have described several forms and several different ways ofcarrying out my invention, I would have it understood that the inventionis not so limited, as it is apparent that various changes andsubstitutions may be made therein and still come within its scope andprinciple; but

What I do claim as my invention, and desire to secure by Letters Patent,is-

1. The combination with a trunk line adapted to connect with a callingtelephoneline at its outgoing end and with a called telephone-line atits incoming end, a signal at the incoming end of the trunk, atrunk-relay at said end actuated by current over the trunk whenconnection is established at the outgoing end for preventing theoperation of said signal, means actuated when the called subscriberresponds for entirely depriving said relay of operating-current, andmeans whereby said relay still remains in operated position althoughdeprived of operating-current through any of its windings and preventsthe actuation of said signal during conversation, substantially asdescribed.

a 2. The combination with a trunk line adapted to connect with a callingtelephoneline at its outgoing end and with a called telephone-line atits incoming end, of a polarized relay at the incoming end of the trunk,a signal at said incoming end whose actuation de pends upon said relay,and means to direct current through said relay in one direction whenconnection is severed at the outgoing end and in the opposite directionwhen con nection is established at said end to effect the desiredoperation of the relay, substantially as described.

3. The combination with a trunk-line, of a calling telephone-line withwhich its outgoing end is adapted to connect and a called telephone-linewith which its incoming end is adapted to connect, of a polarized relayat the incoming end of the trunk, a signal at said incoming end whoseactuation depends upon said relay, and means to direct current throughsaid relay in one direction before connection is established at theoutgoing end and to direct current through the same in the oppositedirection after the connection is established, substantially asdescribed.

4. The combination with a trunk-line, of a calling telephone-line withwhich its outgoing end is adapted to connect and a called telephone-linewith which its incoming end is adapted to connect, of a polarized relayat the incoming end of the trunk, asignal at said incoming end whoseactuation depends upon said relay, and means for normally sendingcurrent through said relay in. one direction and to send it through thesame in the opposite direction by the act of making the connection atthe outgoing end of the trunk, substantially as described.

5. The combination with a trunkline, of a calling telephone-line withwhich its outgoing end is adapted to connect and a called telephone-linewith which its incoming end is adapted to connect, of a polarized relayat the incoming end of the trunk, a signal at said incoming end whoseactuation depends upon said relay, and means for sending current throughsaid relay in one direction when connection is established with theoutgoing end of the trunk and for sending it through said relay in theopposite direction when the said connection is severed, substantially asdescribed.

6. The combinationwith a trunk-line, of a calling telephone-line withwhich its outgoing end is adapted to connect and a called telephone-linewith which its incoming end is adapted to connect, of a source ofcurrent normally connected with the trunk, a trunkrelay connectedbetween the trunk conductors at the incoming end, a signal whoseactuation depends upon said relay, and means for reversing theconnections of the said current source with the trunk when connection isestablished at the outgoing end of the trunk, substantially asdescribed.

7. The combination with a trunk-line, of a calling telephone-line withwhich its outgoing end is adapted to connect and a called telephone-linewith which its incoming end is adapted to connect, of a polarizedtrunk-relay connected between the trunk conductors at the incoming end,a signal whose actuation depends upon said relay, and means forconnecting the live pole of a current source with one of the conductorsbefore connection is established and for connecting the same pole withthe other conductor after connection is established, substantially asdescribed.

8. The combination with a trunk-line, of a calling telephone-line withwhich its outgoing end is adapted to connect and a called telephone-linewith which its incoming end is adapted to connect, of a polarizedtrunk-relay at the incoming end of the trunk, a signal whose actuationdepends upon said relay, a source of current connected with said trunkbefore connection is established therewith at the outgoing end, and arelay actuated when connection is established at said end fordisconnecting said source, substantially as described.

9. The combination with a trunk-line, of a calling telephone-line withwhich its outgoing end is adapted to connect and a called telephone-linewith which its incoming end is adapted. to connect, of a polarizedtrunk-relay connected. at the incoming end of the trunk, a signal whoseactuation depends upon said relay, a source of current at the outgoingend of the trunk connected therewith before connection is establishedwith said end, and a relay actuated when connection is established fordisconnecting said source, substantially as described.

10. The combination with a trunk-line, of a calling telephone-line withwhich its outgo ing end is adapted to connect and a calledtelephone-line with which its incoming end is adapted to connect, of apolarized trunk-relay at the incoming end of the trunk, a signal at saidend whose actuation depends upon said relay, a source of currentconnected with the outgoing end before connection is established andadapted to send current in one direction through said relay, a relayalso connected with the outgoing end of the trunk actuated whenconnection is established for disconnecting said source, and means fordirecting current through said trunk-relay in the reverse direction whenconnection is established at the outgoing end, substantially asdescribed.

11. The combination with a trunk-line, of a calling telephone-line withwhich its outgoing end is adapted to connect and a called telephone-linewith which its incoming end is adapted to connect, of a polarizedtrunkrelay at the incoming end of the trunk, a signal whose actuationdepends upon said relay, a source of current connected with said trunkbefore connection is established and adapted to send current throughsaid relay-in one direction, a relay connected at the outgoing end ofthe trunk, and means for sending an operating-current through said relayto disconnect said source and to also send a current in the reversedirection through said polarized relay when connection is establishedwith the outgoing end of the trunk, substantially as described.

12. The combination with a trunk-line, of a calling telephone-line withwhich its outgoing end is adapted to connect and a called telephone-linewith which its incoming end is adapted to connect, of a polarizedtrunk-relay at the incoming end of the trunk, a signal whose actuationdepends upon said relay, a source of current normally connected with oneside of said trunk, a relay connected with the other side of the trunk,said relay presenting suflicient retardation to prevent short-circuitingof the voice-currents during conversation, a cord-circuit to connectwith the outgoing end of the trunk, a source of current associatedtherewith and means for sending an operatingcurrent from said sourcethrough said relay to open the connection of the first-named source andto direct current in the opposite direction through said trunk-relay,substantially as described.

13. The combination with a trunkline, of a calling telephone-line withwhich its outgoing end. is adapted to connect, and a calledtelephone-line with which its incoming end is adapted to connect, of apolarized trunkre lay at the incoming end of the trunk, a signal whoseactuation depends upon said relay, a normal connection with a source ofcurrent to actuate said relay in one direction, a relay connected withthe other side of said trunk to sever the said connection from saidsource, said relay presenting sufiicient retardation to prevent theshort-circuiting of the voicecur rents during conversation, acord-circuit connected with the outgoing end, a supervisory signalassociated therewith, and means for placing said supervisory signal incondition to operate by current flowing over one side of thetalking-circuit and through said relay, said relay being actuated at thesame time to open said connection, and current being simultaneouslydirected through said polarized relay in the reverse direction tocorrespondingly actuate it, substantially as described.

14. The combination with a trunk-line, of a calling telephone-line withwhich its outgoing end is adapted to connect and a called telephone-linewith which its incoming end is adapted to connect, of a high-resistancepolarized relay at the incoming end of the trunk connected in themetallic circuit thereof, a signal at the incoming end whose actuationdepends upon said relay, a supervisory signal associated with theoutgoing end of the trunk placed in condition to operate when connectionis established therewith, and means for removing the high resistance ofsaid relay from the trunk when the called subscriber responds, whereby asufiicient flow of current is permitted therein to render thesupervisory relay inoperative, substantially as described.

15. The combination with a trunk-line, of

a calling telephone-line with which its outgoing end is adapted toconnect and a called telephone-line with which its incoming end isadapted to connect, of a high-resistance p0- larized trunk-relay at theincoming end of the trunk connected in the metallic circuit of theoutgoing end, a signal at the incoming end whose actuation depends uponsaid relay, a supervisory relay associated with the outgoing end of thetrunk placed in condition to operate by current flowing over themetallic trunk-line and through said high re sistance, and means forremoving said high resistance from the trunk when the called subscriberanswers to permit a sufficient flow "of current therein to render thesupervisory signal inoperative, substantially as described.

16. The combination with a trunk-line, of a calling telephone-line withwhich its outgoing end is adapted to connect and a called telephone-linewith which its incoming end is adapted to connect, of a hi h-resistancepolarized trunk-rel ay connected b etween the trunkconductors at theincoming end, a source of current connected with the trunk and adaptedto send current through said relay in one direction, a signal at theincoming end whose actuation depends upon said relay, a supervisorysignal associated with the outgoing end of the trunk placed in conditionto operate by current flowing over a portion of the talking-circuit andthrough said high-resistance relay and means for removing the resistanceof said relay from between the trunkconductors when the calledsubscriber responds, whereby a sufficient flow of current in the trunkis permitted to render the supervisory signal inoperative, substantiallyas described.

17. The combination with a trunk-line, of a calling telephone-line withwhich its outgoing end is adapted to connect and a called telephone-linewith which its incoming end is adapted to connect, of ahigh-resistance-polarized trunk-relay at the incoming end of thetrunk-circuit thereof, a signal whose actuation depends upon saidtrunk-relay, means for causing current to flow through said relay in onedirection before connection is established with the outgoing end of thetrunk to operate it in one direction, a cord-circuit to connect withsaid end, a supervisory signal associated therewith and placed incondition to operate by current flowing over one side of thetrunkingcircuit, said latter current flowing in the reverse directionthrough said trunk-relay to operate it in the reverse direction, saidhigh-resistance relay serving to limit the flow of current in the trunk,and means for cutting out said high resistance when the calledsubscriber responds to permit an increased flow of current in the trunkto render said supervisory signal inoperative, substantially asdescribed.

18. The combination with a trunk-line adapted to connect with a callingtelephoneline at its outgoing end and with a called tele phone-line atits incoming end, of a disconnect-signal at the incoming end placed incondition to operate when connection is established with the calledline, a polarized relay at the incoming end, and means to operate saidpolarized relay in the proper direction when connection is establishedat the outgoing end of the trunk to prevent the operation of saidsignal, substantially as described.

19. The combination with a trunk-line adapted to connect with a callingtelephoneline at its outgoing end and with a called telephone-line atits incoming end, of a disconnect-signal at the incoming end of thetrunk, means to place said signal in condition to operate whenconnection is established with the called line, a polarized relay at theincoming end of the trunk and means to direct current through said relayin one direction before connection is established at the outgoing endand in the reverse direction when connection is established, the lattercurrent being in the proper direction to prevent the operation of thesignal, substantially as described.

20. The combination with a trunk-line adapted to connect with a callingtelephoneline at its outgoing end and with a called telephone-line atits incoming end, of a disconnect-signal at the incoming end of thetrunk,

and means actuated by current flowing oversaid trunk in one directionwhen connection exists with the called line to prevent the operation ofsaid signal and by current in the reverse direction to actuate saidsignal when the connection is severed at the outgoing end, substantiallyas described.

21. The combination with a trunk-line adapted to connect with a callingtelephoneline at its outgoing end and with a called tele phone-line atits incoming end, of a disconneat-signal at the incoming end, apolarized relay at said end and means to direct current through saidrelay in the proper directionto prevent the operation of said signalwhen a connection is established at the outgoing end, and further meanswhen connection is severed at the outgoing end to direct current throughsaid relay in the reverse direction to suitably operate the same,substantially as described.

22. The combination with a trunk line adapted to connect with a callingtelephoneline at its outgoing end and with a called telephone-line atits incoming end, of a disconnect-signal at the incoming end of thetrunk, a relay actuated over a portion of the talking circuit whenconnection is established with the called line to place said signal incondition to operate, a polarized relay at the incoming end andconnected with the outgoing end of the trunk, said relay being actuatedby current sent through the same in the proper direction when connectionis established at the outgoing end to prevent the operation of saidsignal, substantially as described.

23. The combination with a trunk-line adapted to connect with a callintelephoneline at its outgoing end and with a called telephone-line atits incoming end, of a disconnect-signal at the incoming end of thetrunk, a relay actuated over a portion of the talking-circuit whenconnection is established with the called line to place said signal incondition to operate, a polarized relay at the incoming end andconnected with the outgoing end of the trunk, and means to directcurrent through said polarized relay when connection is established atthe outgoing end, said current being in a direction to actuate the relayso as to prevent the said signal from operating, and further meansactuated upon severing the connection at the outgoing end, said currentbeing in a reverse direction to actuate said relay so as to cause saidsignal to operate, substantially as described.

24. The combination with a trunk-line adapted to connect with a callingtelephoneline at its outgoing end'and with a called telephone-line atits incoming end, of a disconnect-signal for the incoming end of thetrunk, a relay actuated over one side of the talking-clrcuit whenconnection is established with the called line to place said signal incondition to operate, a polarized trunk-re lay at the incoming end ofthe trunk and connected with the outgoing end, a source of currentconnected with said trunk to normally send a current through the relayin a direction to actuate the same so as to permit said signal tooperate and means for directing current over the trunk in'the reversedirection when a connection is established at the outgoing end tooperate said relay in the reverse direction and prevent said signal fromoperating, substantially as described.

25. The combination with a trunk-line adapted to connect with a callingtelephoneline at its outgoing end and with a called telephone-line atits incoming end, of a disconnect-signal at the incoming end of thetrunk, a relay actuated over one side of the talking-circuit to placesaid signal in condition to operate, a polarized trunk-relay at theincoming end of the trunk and connected with the outgoing end, a sourceof current associated with the outgoing end and connected with the trunkwhen a conversational circuit is established to direct current throughsaid polarized relay in a suitable direction to prevent the operation ofsaid signal, and a relay responsive to current in the called lineadapted to deprive said trunkrelay of operating current duringconversation, the armature of said polarized relay remaining in theposition to which it was last moved during the time the trunk-relay isdeprived of operating-current whereby the said signal ismaintained-inert, substantially as described.

26. The combination with a trunk-line adapted to connect with a callingtelephoneline at its outgoing end and with a called telephone-line atits incoming end, of a disconnect-signal at the incoming end of thetrunk, a relay actuated over one side of the talking circuit whenconnection is established with the called line to place said signal incondition to operate, a polarized relay at said incoming end adapted toreceive current in a suitable direction when a connection is firstestablished at the outgoing end to' actuate the armature thereof in suchmanner as to prevent the operation of said signal, means for deprivingthe same of operating-current upon the response of the calledsubscriber, the armature of said trunk-relay remaining in the positionto which it was last moved during the time the relay is deprived ofoperating-current whereby the said signal remains inert, substantiallyas described.

27. The combination with a trunk-line adapted to connect with a callingtelephoneline at its outgoing end and with a called telephone-line atits incoming end, of a guardsignal at the incoming end and connected atthe outgoing end, a source of current normally connected with the trunkto direct current through said relay so as to actuate it in such manneras to prevent the normal operation of said signal, and means todisconnect said source and direct current through said relay in thereverse direction when a connection is established at the outgoing endto actuate said relay in the proper direction to operate said signal,substantially as described.

28. The. combination with a trunkline adapted to connect with a callingtelephoneline at its outgoing end and with a called telephone-line atits incoming end, of a signal at the incoming end of the trunk, a relayadapted to be actuated over one side of the talkingcircuit whenconnection is established with the called line, a polarized trunk-relayat said incoming end, said signal being actuated to give a guardindication when the firstnamed relay is deenergized and the polarizedrelay is actuated in one direction by current sent over the trunk whenthe connection is established at the outgoing end, substantially asdescribed.-

29. The combination with a trunk line adapted to connect with a callingtelephoneline at its outgoing end and with a called telephone-line atits incoming end, of a signal at the incoming end of the trunk, a relayactuated over one side of the talking-circuit when connection isestablished with the called line,

- a polarized relay at the incoming end of the trunk, said signal beingcon trolled by said two relays so as to give a guard indication when aconnection exists at the outgoing end and before connection isestablished at the incoming end and to give a disconnect indication whenconnection exists at the incoming end and is severed at the outgoingend, substantially as described.

30. The combination with a trunk line adapted to connect with a callingtelephoneline at its outgoing end and with a called telephone-line atits incoming end, of a signal at the incoming end of the trunk, a relayactuated over one side of the talking-circuit when connection isestablished with the called line, a polarized relay at the incoming endof the trunk, said signal being entirely controlled by said two relaysto operate the same to give a guard indication when the first-namedrelay is deenergized and the second is actu ated by current over thetrunk when the connection exists at the outgoing end only and to give adisconnect indication when the firstnamed relay is energized and thepolarized relay is actuated by current flowing over the trunk when theconnection has been severed at the outgoing end, substantially asdescribed.

31. The combination with a trunk-line, of a cord-circuit to connect withits outgoing end and a subscribers line with which its in coming end isadapted to be connected, a supervisory signal associated with thecord-circuit and placed in condition to operate by current flowing overa portion of the talkingcircuit when the cord is connected with thetrunk, a relay associated with the incoming end of the trunk and adaptedto be operated over a portion of the talking-circuit when the trunk isconnected with the called line, a testing-circuit for said incoming endof the trunk normally completed over one strand thereof and through thenormal contacts of said relay, said testing-circuit being severed andthe trunk-circuit being placed in condition for talking by the actuationof said. relay, a ringing signal for the incoming end of the trunkplaced in condition to operate by the actuation of said relay, means forrendering inert and for locking out said ringing-signal during theremainder of the connection, a polarized trunlcrelay at the incoming endof the trunk actuated in one direction by current over the trunk fromthe cord-circuit when the cordcircuit is connected with the trunk, meansto permit the flow of current over the metallic trunk-line to rendersaid. supervisory signal inert when the called subscriber responds, asecond signal at said incoming end of the trunk placed in condition tooperate by the actuation of the first-named relay, means for operatingsaid signal when the cord-circuit is disconnected from the trunk and thetrunk is connected with the subscribers line, whereby a disconnectindication is given, and further means for actuating said signal whenthe cord is connected with the trunk and the trunk is disconnected fromthe trunk-line, whereby a guard indication is given, a source of currentto furnish current for the operation of the relays and signals and tocharge the transmitters for talking purposes, substantially asdescribed.

32. The combination with a trunk line adapted to connect with a callingtelephoneline at its outgoing end and with a called telephone-line atits incoming end, a trunk-relay at the incoming end of the trunk ofrelatively high resistance and connected in series in the metalliccircuit of the outgoing end, a source of current and a relay normallyconnected in series in said metallic circuit at the outgoing end, theresistance of said trunk-relay being such as to prevent a suflicientflow of current in said metallic circuit to operate said second relay,substantially as described.

33. The combination with a trunk line adapted to connect with a callingtelephone line at its outgoing end and with a called telephone-line atits incoming end, a trunk-relay at the incoming end of the trunk ofrelatively high resistance and connected in series in the metalliccircuit of the outgoing end, a source of current and a relay normallyconnected in series in said metallic circuit at the outgoing end, theresistance of said trunk-relay being such as to prevent a suflicientflow of current in said metallic circuit to operate said second relay,said second relay being operated during a connection at the outgoingendof the trunk to open the normal connection to said source, substantiallyas described.

34. The combination with a trunk line adapted to connect with a callingtelephoneline at its outgoing end and with a called telephone-line atits incoming end, a polarized trunk-relay at the incoming end of thetrunk of relatively high resistance and connected in series in themetallic circuit of the outgoing end, a source of current and a relaynormally connected in series in said metallic circuit at the outgoingend, the resistance of said trunkrelay being such as to prevent asufficient flow of current in said metallic circuit to operate saidsecond relay, said second relay being operated during a connection atthe outgoing end of the trunk to open the normal connection to saidsource, substantially as described.

35. The combination with a trunk-line, of a cord-circuit to connect withits outgoing end and a subscribers line with which its incoming end isadapted to be connected, a re lay legged to ground from one side of thetalking-circuit established by the connection of the cord-circuit withthe trunk, said relay having sufficient impedance to prevent the passageof voice-currents, a supervisory signal associated with the cord circuitand placed in condition to operate by current flowing over a portion ofthe talking-circuit and through said relay when the cord is con nectedwith the trunk, a source of current normally connected with the otherside of the trunk-circuit at the outgoing end and adapted to bedisconnected by the operation of said relay, a polarized trunk-relay atthe incoming end of the trunk connected with said outgoing end andactuated in one direction by current from said source under normalconditions and in the opposite direction by current from thecord-circuit when the latter is connected with the trunk, a source ofcurrent at the incoming end, a relay also associated with the incomingend of the trunk and adapted to be operated by current from said sourceover one strand when connection is established with the called line,

said relay serving when actuated to place the trunk in condition forconversation, a ringing-signal for the incoming end of the trunk placedin condition to operate by the actuation of said relay, a second relayresponsive to current in the trunk from said source when the calledsubscriber answers his call, said latter relay serving to render thesaid ringing-signal inert, means for looking out said ringing-signalduring the remainder of the connection, means operated by the saidsecond relay to permit a flow of current over the metallic trunk-line torender said supervisory signal inert when the called subscriberresponds, a second signal at said incoming end of the trunk placed incondition to operate by the actuation of the first-named relay, saidsignal being actuated when the cord-circuit is disconnected from thetrunk, and the trunk is connected with the called-subscribers line andthe called-subscribers line is not in use, whereby a disconnectionindication is given, said signal being also operated when thecord-circuit is connected with the trunk and the trunk is disconnectedfrom the called line to give a guard indication, said source servingalso to charge the,transmitters for talking as well as for the operationof said relays and signals, substantially as described.

36. The combination with a trunk-circuit extending between differentswitchboardsections and adapted to interconnect telephone-lines forconversation, of a polarized relay bridged across the circuit and havinga high resistance, a signal associated therewith whose actuation dependsupon'said relay, a source of current normally connected at the oppositeend of the trunk and adapted to send current through said relay tooperate it in one direction, a cut-off relay for said end of the trunk,and means when connection is established therewith to sever saidconnection and to send current in the reverse direction through saidrelay, substantially as described. V

Signed by me at Chicago, county of Cook, State of Illinois, this 12thday of June, 1902.

WILLIAM W. DEAN. Witnesses:

ROBERT LEWIS AMEs, G. BEDER.

